Field of the Invention
The field of the invention is data processing, or, more specifically, methods, apparatus, and products for multipath I/O in a computer system.
Description of Related Art
Many computer systems allow multiple physical paths between a host and attached I/O (input/output) devices, such as storage devices. For example, a disk storage device may be accessible to a host via either of a pair of Fibre Channel adapters. In another example, a virtual disk storage device may be accessible via either of a pair of virtual I/O servers.
When issuing an I/O request, a host in such a multipath environment may consider the set of paths available and choose a single path to service the I/O request. The host may take into account various attributes of the paths when selecting the path to service the I/O request. Such attributes may include, for example, a number of I/O requests currently outstanding for each path, recent I/O response times of each path, recent I/O failures of each path, and the like.
Even with consideration of such attributes however, a host is still exposed to the possibility that an I/O request issued to a ‘best expected’ path may be delayed. For example, a spike in activity on a Fibre Channel adapter or in a virtual I/O server may cause a delay in initiating an I/O request. Some other problem in the adapter of virtual I/O server may cause the I/O to fail entirely, but in a way that is detected only after a lengthy time period.
In cases where an I/O request along one path fails after a delay, the host may recover by retrying the I/O request along a second path. Such a “retry after delayed failure” can extend the service time of an I/O request by several orders of magnitude.